One of the primary literary techniques (an optional literary device) used by Dickens in constructing Oliver Twist is narratorial voice. Dickens takes several approaches to narratorial voice that affect the tone and the mood (atmosphere) of the novel.
For instance, in the opening chapter, Dickens sets the tone as he establishes a narrator who is at a close distance and who will comment freely on the story throughout the story:
if, during this brief period, Oliver had been surrounded by careful grandmothers, anxious aunts, experienced nurses, and doctors of profound wisdom, he would most inevitably and indubitably have been killed in no time.
Dickens also establishes the ironic tone that he uses often throughout the story. This ironic tone is apparent in the quote above and is further apparent in this:
The medical gentleman walked away to dinner; and the nurse, having once more applied herself to the green bottle, sat down on a low chair before the fire, and proceeded to dress the infant.
Dickens further adds to the tone as he establishes that the narrator will make direct comments in a serious tone on social evils:
But now that he was enveloped in the old calico robes which had grown yellow in the same service, he was badged and ticketed, and fell into his place at once- a parish child- the orphan of a workhouse- the humble, half-starved drudge- to be cuffed and buffeted through the world- despised by all, and pitied by none.
The mood (atmosphere) of a work of literature is a combination of diction, imagery, setting, and details of the scene. The narrator adds to the atmosphere by his diction and vocabulary. For instance, the lightly bantering mood of Chapter 1 is nowhere to be seen in Chapter 48 in which the mood changes to one of somber foreboding:
Of all bad deeds that, under cover of the darkness, had been committed within wide London's bounds since night hung over it, that was the worst. Of all the horrors that rose with an ill scent upon the morning air, that was the foulest and most cruel.
Dickens' diction is formal. His vocabulary is full of gloomy, foreboding words bad, under cover,darkness, worst, foulest, horrors. The mood is nothing like the ambling pleasant mood in Chapter 1.